It Looks like a problem of version 2.10 of the wrapper version on gradle.
After reading around people's workaround like
- "invalidate cache and restart"
- delete .gradle file in the project and re-open it
- etc etc
I just try for fun to install a standalone version of gradle since I'd always used only the wrapper version that cames with android studio.
And with it there is any error and any warning.
For anyone who wanna try (I'm a mac's user):
- install gradle (by homebrew is fine and pretty easy) brew install gradle
- tell to AS to use that one instead the wrapper version preference -> build, Execution, Deveployment -> Gradle -> use local gradle distribution
if you install it by homebrew like me the correct path will be: /usr/local/Cellar/gradle/x.yy/libexec
where x.yy it suppose to be 2.11 (that is the last version provide by brew even if the lastest version released is 2.13)
I've realised just now that the steps above doens't fix the definitly. Each time you re-open the project the problem came again and again.
It's annoying but changing betwen use of "wrapper"/"regular gradle" take the warning away.
But this means also that isn't a problem o the gradle as I'd assumed at the beginning.
...let's wait a permanent fix ^^